What is the required number of witnesses to establish the punishment for anal intercourse (watu' fi al-dubar)?

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Anal intercourse is deemed a grave immorality (fahisha), evidenced by the saying of Allah: {Do you approach men with lust rather than women? Nay, you are a people transgressing} (Quran 7:80). Furthermore, the verse regarding immoral conduct by women states: {And those who commit immorality from among your women, call to witness against them four from among you} (Quran 4:15). Therefore, if a woman is subjected to anal intercourse, it falls under the generality of this verse, requiring four witnesses.